This invention relates to interior permanent magnet motors of the kind comprising a laminated rotor core which includes pockets into which permanent magnets are located. It also relates to rotor structures for such motors and a method of fabricating a rotor structure.
Interior permanent magnet motors are known. A rotor structure of a typical prior art interior permanent magnet motor 1 is shown in FIG. 1 of the drawings. It comprises a rotor structure that connects to an output shaft 2 from which the output of the motor is taken. The rotor structure comprises a rotor core 3 formed from a stack of laminated plates. The core 3 has a central opening 4 that extends from a first end of the core to a second, opposing, end of the core and this enables it to be fixed around the shaft. The core in the example of FIG. 1 comprises three pucks 5, each comprising a stack of substantially identical thin metal plates 6 which are pressed together. The core may comprise just one, or perhaps two or more, pucks, the pucks being fixed together end to end form the complete stack.
Each of the plates 6 in the stack includes at least one opening or window 7. In the example there are eight of these openings, spaced evenly around the axis of the plate. The plates are held in the stack so that the windows in a plate align with corresponding openings in the adjacent plates as can be seen in FIG. 2. Each set of windows, from one end of a puck 6 to the other, forms a pocket 9. In an interior permanent magnet motor each pocket contains an elongate rare earth magnet 10, the shape of which is generally complimentary to the pocket so that it is a snug, but not interference, fit. The magnets are typically retained in the pockets with glue. Generally the length of the magnet is the same as the length of the stack, so that a first end of the magnet is aligned with a first end of the core and a second end of the magnet is aligned with a second end of the core. This gives the optimum magnetic properties for the rotor structure. In other arrangements, as shown in FIG. 2, they may be retained by adding an extra plate 11 to each end of a puck which has no windows, the plate sitting over the end of each magnet so that the laminated stack is slightly longer than the magnets.